1. Field of the Invention
This invention relates to an optical deflector having a deflection means for deflecting light.
2. Related Background Art
FIG. 1 of the accompanying drawings illustrates a galvano-mirror as an example of optical deflector that is driven by electromagnetic force. A mirror is arranged on a movable section, which is supported by a main body by way of a pair of torsion bars so that it may be rotated relative to a central axis. In FIG. 1, reference symbol 50 denotes a silicon substrate and reference symbols 51 and 52 respectively denote an upper glass substrate and a lower glass substrate. There are also shown a movable plate 53, a pair of torsion bars 54, a planar coil 55, a total reflection mirror 56, a pair of electrode terminals 57 and permanent magnets 60 through 63. The illustrated optical deflector is of the electromagnetic type that is driven by causing a drive current to flow through the planar coil 55 and utilizing the Lorentz force that is generated by the drive current and the permanent magnets (see, inter alia, U.S. Pat. No. 5,606,447).
Japanese Patent Application Laid-Open No. 2001-305471 describes an electromagnetic actuator. This patent document has much in common with U.S. Pat. No. 5,606,447 in that a movable part is driven by electromagnetic force. The electromagnetic actuator disclosed in Japanese Patent Application Laid-Open No. 2001-305471 also has a total reflection mirror arranged on a movable part.
Japanese Patent Application Laid-Open No. 2001-305471 describes a system as follows in terms of problems, objects and means. The invention disclosed in the above patent document paid attention to the fact that the resonance period of an electromagnetic actuator normally drifts with temperature and time and hence, if an electric current having a predetermined, constant resonance frequency is continuously supplied to the planar coil, there arises a problem that it is not possible to control the angle of deflection and keep it to a constant value with temperature change and time lapse. Thus, the first object of that invention is to provide an electromagnetic actuator that can be driven to reciprocate with its resonance period without providing a separate detection means as well as a drive control device and a drive control method to be used for such an electromagnetic actuator. The second object of that invention is to provide an electromagnetic actuator whose angle of deflection can be controlled without providing a separate detection means as well as a drive control device and a drive control method to be used for such an electromagnetic actuator. The third object of the invention is to provide a resonance frequency signal generating device and a resonance frequency signal generating method to be used for an electromagnetic actuator that can output a resonance frequency signal corresponding to the resonance period of the electromagnetic actuator.
The invention of the above cited patent document utilizes a coil as means for exciting the movable section of the electromagnetic actuator and also as detection means. The induced voltage or current in the coil is utilized for detection.
While Japanese Patent Application Laid-Open No. 2001-305471 describes that the resonance period of an electromagnetic actuator drifts with temperature and time, it proposes to detect the time when the actuator passes by a predetermined angle of revolution (deflection) from the coil that is a detection means as time-related information.
U.S. Pat. No. 5,606,447 does not pay attention to the problem that the resonance period of an electromagnetic actuator drifts with temperature.
With the method of detecting the time when the actuator passes by a predetermined angle of deflection gives rise to a signal delay in the detection circuit due to changes of environmental temperature. Additionally, timing errors can occur when gauging the change with time of the angle of deflection and detecting the time on the basis of the gauged change because the detection timing can be shifted by signal noises and offsets.
Thus, the problem of signal delays and timing errors arises when accurately controlling an actuator by such a method of detecting the time when the actuator passes by a predetermined angle of deflection.